Usually I would if it looked better at all. Besides, I don't think I've ever noticed smooth shading on any of the original items besides the little circles on the printer. I just think it usually looks better to just flat-shade it.
3 of the hats, 4 of the packs, shoulder plates, shoulders, jeep, rocket launcher, bow, wand, spear, and horse all have some smooth shading.
You just have to know where and when to smooth shade.